Minutes after he took a seat among hundreds of religious scholars at a ceremony for the Prophet Mohammad's birthday in Kabul, a huge explosion shook the room, throwing Ahmad Fareed to the floor.

Scattered around him were the bloodied bodies of turbaned men who had taken the full force of the suicide blast and probably saved his life.

"It was absolute horror, people dead and wounded, covered in blood and pieces of flesh," Fareed, 40, told AFP on Wednesday as he lay in a hospital bed with leg and shoulder injuries.

"My friend and his little son were also next to me, covered in blood and not moving."

At least 55 people were killed and 94 wounded when the bomber blew himself up in the middle of the banquet hall at the Uranus Wedding Palace on Tuesday evening - one of the deadliest attacks in Afghanistan this year.

A video posted on social media purportedly filmed by one of the hundreds of attendees showed a large gathering of men listening to a recitation of the Koran before an explosion triggered pandemonium.

On Wednesday morning, as cleaners prepared the multi-story venue for more events, bloodied turbans, sandals, overturned chairs and broken glass still littered the cavernous room where the massacre happened.

The attack drew widespread condemnation in Afghanistan and abroad, with the United Nations describing it as an "atrocity."

US Ambassador to Kabul John Bass said he was "sickened and deeply saddened" by the bombing, while the European Union delegation to Afghanistan said the explosion was "an attack on all of us, religious or not, who value freedom."

President Ashraf Ghani declared Wednesday a national day of mourning for the victims of the attack, which he described as an "unforgivable crime."

Security at the wedding hall included a body search by an armed guard followed by physical checks by several religious students, said Obaidullah, who suffered multiple injuries to his face and body.